Carpets are generally produced by tufting carpet yarns into a primary backing, the tufts being secured in the primary backing through use of a "precoat" or "tuft bind" adhesive. The greige good thus prepared may optionally be coated with adhesive and secured to a secondary backing, generally of polypropylene or jute. Key properties of the carpeting produced by these processes are tuft bind, the strength required to pull tufts from the primary backing, and delamination strength, the force required to separate the secondary backing from the carpet.
High values of tuft bind are obtained from high strength adhesives which thoroughly wet out both the tufts and the primary backing. In order to achieve suitable delamination strength, the secondary backing must be in intimate contact with the tufted yarn/primary backing ("greige good") during the curing process, and a sufficient amount of adhesive which ensures complete wet out of the tufted fibers and secondary backing and which promote a good marriage between these components must be used.
In the past, SBR latex has been almost exclusively used as both the tuft bind adhesive and the laminating adhesive. The latex may be applied by numerous methods, such as the indirect applicator (pan application) which is now used infrequently, and "direct" methods, these involving roll over flatbed or roll over roll processes, either as a single unitary process, or in series as a tandem process. Regardless of the actual method used, the reverse of the greige good is coated with a precoat of adhesive, and the secondary backing, also coated with adhesive, is married to the greige good, and cured. In these processes, the carpets are coated face down, but cured "face up", i.e., with the aesthetic side of the carpet uppermost.
While SBR latex has been the adhesive of choice, carpet prepared from SBR latex also exhibits numerous deficiencies. For example, the tensile strength and hydrolytic stability of SBR latexes is less than desired, and SBR polymers are less durable than desired. Greige goods in which SBR latexes are used as tuft bind adhesives are subject to "rewicking". Moreover, use of SBR latexes requires the evaporation of large quantities of water during cure, a process which is energy intensive. Polyurethane adhesives are capable of forming a laminate with superior tuft bind and delamination strength, as well as improving other desirable physical properties. However, technical problems have prevented widespread use of polyurethanes despite the increased performance potential.
Substitution of reactive polyurethane systems for latex laminating adhesives has not been straight forward, and has required numerous process modifications. For example, conventional latex maintains its tackiness and viscosity during processing, even in the curing oven. Following application of latex adhesive, the back-side of the greige goods is faced downward. A coat of additional latex is applied to the secondary backing by a skip roll. As a result of the latex properties, good temporary adherence of the secondary backing to the greige goods is obtained. In the curing oven, the latex viscosity does not drop significantly as water evaporates. Thus, the secondary backing satisfactorily adheres to the greige goods, and dripping of latex from the bottom of the laminate into the oven does not occur.
With polyurethanes, however, the single adhesive application common with latex has not been found operable. Polyurethane froth adhesives are not as inherently tacky as SBR latex. Following application of polyurethane adhesive to the greige good and the working of the adhesive into the tufts by means of a closely spaced doctor bar or similar device, a considerable additional amount of polyurethane must be applied from a second puddle in order to provide enough adhesive to temporarily adhere the secondary backing. Even with this additional adhesive, the slow advancement of the adhesive and its low level of tack does not allow for the desired adherence between the greige goods and secondary backing.
In the curing oven, polyurethanes generally exhibit a considerable initial decrease in viscosity prior to cure. As a result, unless the carpet laminating line is run with the greige goods reverse side uppermost, considerable loss of polyurethane into the curing oven would occur. Moreover, if sufficient initial adherence of secondary backing to the greige good has not been obtained, separation of the secondary backing may occur during this period of low viscosity. The viscosity of the polyurethane adhesive may decrease to only 10% of its initial value prior to the viscosity rising as the catalyzed urethane-forming reaction begins to exert its effect. The period of greatest viscosity decrease is often exhibited over the temperature range of ambient to 70.degree. C. where the polyurethane catalysts are not optimally active.
Due to the decrease in viscosity exhibited by polyurethane adhesives, carpets employing polyurethane adhesives cannot be cured "face up" with or without a secondary backing. The low viscosity allows the adhesive to drip to the floor of the oven or oven belt, eventually creating a buildup which must be removed; causing smoke and odor to be produced; and creating a risk of fire. Addition of secondary backing can minimize the dripping, but generally does not alleviate it altogether. Moreover, the low viscosity of the adhesive may allow the secondary backing to sag and pull away from the carpet, and migration of low viscosity polyurethane through the relatively large holes of the backing create drips and runs which render the carpet commercially unacceptable. These problems have proven to be major obstacles to acceptance of polyurethane adhesives in carpet manufacturing. The capital investment required to convert conventional latex adhesive lines to polyurethane adhesive lines, where the carpet runs face down in the oven, is considerable.
Simply raising the viscosity of the reactive polyurethane adhesive, for example by employing more viscous polyols or prepolymers can assist in lowering dripping and running. However, the higher viscosity impedes impregnation and wet-out of tuft ends and primary backing. Tuft bind then suffers considerably.